As is known in the art, in telecommunications systems, satellite systems and other systems, it is desirable for radio frequency (RF) power amplifiers (PAs) to linearly amplify RF signals in a highly efficient manner. Efficiency is generally proportional to input drive level. High efficiency is typically not attained until an amplifier approaches its maximum output power. This, however, is not consistent with linear operation. Thus, a tradeoff must typically be made between achieving maximum efficiency and high linearity in RF power amplifier circuits. Furthermore, an RF amplifier that compresses its input or has a non-linear input/output relationship may cause the output signal to spread onto adjacent radio frequencies. This can be a source of interference on other RF channels.
As is also known in the art, predistortion is a technique used to improve the linearity of RF transmitter amplifiers. Predistortion can be implemented using either analog and/or digital techniques.
In general, a predistortion circuit inversely models the amplifier's non-linear gain and phase characteristics and, when combined with the amplifier, produces an overall system that is more linear and reduces the amplifier's distortion. In essence, “inverse distortion” is introduced into the input of the amplifier, thereby reducing (or ideally, cancelling) any inherent non-linearity the amplifier might have.
Since RF power amplifiers tend to become more non-linear as their power increases towards their maximum rated output, predistortion is a technique which allows one to obtain additional usable RF power from an RF amplifier without having to utilize a larger amplifier. Since the cost of RF amplifiers typically rises as a function of the maximum RF power rating of the RF amplifier, in addition to improving performance, predistortion is also a cost-saving technique since utilizing predistortion increase a maximum RF power rating of an RF amplifier.
As is also known, the need for data-intensive communications (e.g., digital video) for warfighters deployed in the field has created a requirement for RF transmitters having an instantaneous RF bandwidth of 20 MHz or more. Concurrently, there is a demand for improved efficiency in RF front-ends to increase battery life, reduce weight, and reduce operating cost. Achieving an acceptable efficiency over a necessary modulation bandwidth has proven to be a difficult challenge.
Referring now to FIG. 1, one state-of-the-art method for improving the effectiveness of communication transmitters without sacrificing efficiency is utilizing envelope-tracking technology. As illustrated in FIG. 1, an envelope-tracking RF amplifier circuit includes an amplifier 12 having a drain modulation circuit 14 coupled thereto. Drain modulation circuit 14 changes the drain voltage provided to transmitter power amplifier 12 in time in accordance with the amplitude of a modulated RF signal 16 as shown in FIG. 1. Envelope-tracking transmitters ideally minimize wasted DC power and exhibit much higher efficiency compared to conventional, fixed-drain power amplifiers.
In state-of-the-art envelope-tracking transmitters, the instantaneous RF bandwidth is limited to the bandwidth of the drain modulator which is typically in the range or about 5 MHz to about 10 MHz. While this is sufficient for some applications (e.g. current warfighter needs), it will soon be superseded by future demands and the ability to achieve 20 MHz or more bandwidth at high power and high efficiency is desired. In order to meet these requirements, it is necessary to generate a clean, accurate envelope. This becomes exceedingly difficult and impractical, for example, when designing around extensive selection of military waveforms with high crest factors.